1. Field of Invention
The invention relates to a device for introduction into a human body for the purpose of measuring and/or monitoring physiological parameters such as pressure, pressure waveform, or fluid flow rate. This invention is particularly useful for chronic monitoring of physiological parameters within the sac of an aneurysm.
2. Description of Prior Art
In the arteries of human bodies one of the major problems is the loss of strength of the wall of arteries, which can result in aneurysm formation. An aneurysm may endanger the health of a patient because of the risk of internal bleeding which often results in the death of the patient. Therefore, aneurysms are often treated before rupture occurs by minimally invasive implantation of devices such as endovascular prostheses, sometimes called stent-grafts or endoprotheses, which isolate the aneurysm sac from fluid conveyance and thus exclude the aneurysm sac from the circulation.
Such endoprostheses are well known to the person skilled in the art. Although the procedure can be safely performed now, endoleakage is still a major problem immediately following the operation as well as many months or years following the procedure. Endoleakage is the incomplete sealing of the aneurysm sac by the endoprosthesis. Because the aneurysm sac is isolated from fluid conveyance by the endoprosthesis, inflowing blood has no exit path, resulting in a pressure build-up within the aneurysm sac. Monitoring endoleakage is usually performed by non-invasive visualization of the endoleakage by, for example, a CT-scan magnetic resonance, duplex ultrasound, and the like. These non-invasive methods require the patient to frequently return to a hospital or imaging center at substantial expense. In addition, the patient may suffer from a ruptured aneurysm sac caused by an endoleak that develops or worsens between non-invasive monitoring periods. Finally, because of limitations with these current non-invasive monitoring systems, failure to visualize an endoleak does not exclude the presence of such an endoleak. Thus, without a visible endoleak, the aneurysm sac can still be under pressure with the danger of ultimate rupture and internal bleeding.
Baum et. al. in the Journal of Vascular Surgery Vol. 33 No. 1 pages 32-41 incorporated herein by reference describes a translumbar sac puncture technique for measuring aneurysm sac pressure. Baum describes accessing the aneurysm sac with a needle and a 5 Fr (1.7 mm internal diameter) sheath through the patient's back. The needle is removed and the aneurysm sac pressure measured through the sheath lumen with a pressure transducer. This technique suffers from several drawbacks:
First, Baum is invasive and must be performed by a physician in a clinical setting. This requires the patient to return to the clinic on a regular basis (Baum suggests every six months on page 39) similar to the non-invasive monitoring schedule described above.
Second, the pressure transducer or sensor (both terms will be considered synonymous throughout) Baum describes must remain outside the patient so that the transducer can be “zeroed” to compensate for atmospheric pressure and periodically calibrated to account for time-dependent drift.
U.S. Pat. No. 6,159,156 to van Bockel discloses a device comprised of a pressure sensor, transducer, telemetry system, and power source implanted entirely within the aneurysm sac through an arterial catheter. However, van Bockel imposes several limitations:
First, the device van Bockel describes is energized using a magnetic or electrical field. Energizing the device in this fashion would require the patient with to charge it frequently from a fixed base station, this would be very inconvenient. In addition, many patients have co-morbidities that require additional implants such as a pacemaker or defibrillator. These devices are known to be sensitive to the magnetic and electrical fields required to energize the transducer.
Second, if a battery were used, the small size van Bockel describes would severely limit the life of the device, requiring frequent battery replacement over the patient's lifetime. Since the device is entrapped between the endoprosthesis and the aneurysm artery wall, replacing the battery would involve an open-abdomen surgical procedure, negating the advantage of the original endovascular procedure.
Third, the small size of the van Bockel's device necessarily limits the memory capacity, the gain of the transducer, and the transmission strength of the transponder.
Fourth, the van Bockel's device must be implanted into the aneurysm before the endoprosthesis is implanted, allowing the opportunity for the device to dislodge from the aneurysm sac.
Fifth, because van Bockel's device is entrapped within the aneurysm sac, it cannot be removed or replaced if it malfunctions, becomes fouled with thrombus (clotted blood), or needs to be calibrated, without an open-abdomen surgical procedure.
Finally, the device disclosed by van Bockel has no means for compensating for changes in atmospheric pressure (so-called “zeroing” the transducer). Since the pressure measurement in question may be on the order of 20 mm Hg (0.4 PSI or 2.67*103 Pa), failure to account for atmospheric pressure would make the error a large percentage of the pressure measurement.
U.S. Pat. No. 5,967,986 to Cimochowski; U.S. Pat. No. 6,237,398 B1 and U.S. Pat. No. 6,277,078 B1 to both Porat; all three incorporated herein by reference, disclose devices for measuring pressure and flow attached to stents and stent-grafts. However, both Cimochowski and Porat disclose devices with sensors placed within the blood flow, rather than within an aneurysm sac. Thus, these devices would be of little use for monitoring aneurysm sac pressure.
U.S. Pat. No. 6,033,366 to Brockway discloses an implantable pressure measurement device consisting of a fluid-filled catheter within an artery connected to a sealed electronics package outside the artery. The electronics package contains a transducer, signal processing chip, telemetry circuit, and power source. Although useful in a laboratory setting in an animal model, the Brockway's device would suffer from several limitations if used to measure aneurysm sac pressure:
First, the Brockway requires a surgical procedure (see also U.S. Pat. No. 4,846,191 Brockway 1989) to be implanted. Although possible in an animal model, in a human implantation of the device as Brockway describes would require an open-abdomen surgical procedure. In U.S. Pat. No. 4,846,191 Brockway describes implanting the catheter of his device into the descending aorta through a femoral access, followed by subcutaneous implantation of the housing. However, this would not be practical for pressure measurements in a human aneurysm sac because it would require placing the catheter between the endoprosthesis and the aorta, which itself would be a source of endoleak. In addition, the femoral access Brockway describes requires ligation of the femoral artery. In a human patient, this would lead to loss of the lower portion of the ligated limb.
Second, Brockway accounts for atmospheric pressure by using a second, external device (i.e. outside the patient's body) to measure the atmospheric pressure and perform the appropriate subtraction. Therefore, the device could provide an accurate measurement only when the patient is in proximity to the external device, assuming that the external device was calibrated frequently to account for shifts in the atmospheric pressure caused by changing weather patterns.
Third, the Brockway has no means of calibration to account for drift in the transducer except to surgically remove the transducer and replace it with a new transducer.
U.S. Pat. No. 6,106,476 to Corl incorporated herein by reference discloses a miniature pressure sensor mounted on a 0.014-0.018″ wire. As Corl depicts in FIG. 1 of U.S. Pat. No. 6,106,476; his device is not intended as a chronic implant because, as Brockway teaches in U.S. Pat. No. 4,846,191; devices based on miniature solid state sensors such as Corl's “require calibration within a short time prior to use and are suitable only for acute measurements”. Corl's pressure sensing wire also would be unable to account for variations in atmospheric pressure unless part of the device remained outside the body as he depicts in '476. Therefore, while providing benefits over Brockway such as reduced diameter and kink resistance, Corl's device would not be suitable for chronic measurement of pressure within an aneurysm sac.